In the year 2000, an estimated 22 million people were suffering from cancer worldwide and 6.2 millions deaths were attributed to this class of diseases. Every year, there are over 10 million new cases and this estimate is expected to grow by 50% over the next 15 years (WHO, World Cancer Report. Bernard W. Stewart and Paul Kleihues, eds. IARC Press, Lyon, 2003). Current cancer treatments are limited to invasive surgery, radiation therapy and chemotherapy, all of which cause either potentially severe side-effects, non-specific toxicity and/or traumatizing changes to ones body image and/or quality of life. Cancer can become refractory to chemotherapy reducing further treatment options and likelihood of success. The prognosis for some cancer is worse than for others and some are almost always fatal. In addition, some cancers with a relatively high treatment success rate remain major killers due to their high incidence rates.
One of the causes for the inadequacy of current cancer treatments is their lack of selectivity for affected tissues and cells. Surgical resection always involves the removal of apparently normal tissue as a “safety margin” which can increase morbidity and risk of complications. It also always removes some of the healthy tissue that may be interspersed with tumor cells and that could potentially maintain or restore the function of the affected organ or tissue. Radiation and chemotherapy will kill or damage many normal cells due to their non-specific mode of action. This can result in serious side-effects such as severe nausea, weight loss and reduced stamina, loss of hair etc., as well as increasing the risk of developing secondary cancer later in life. Treatment with greater selectivity for cancer cells would leave normal cells unharmed thus improving outcome, side-effect profile and quality of life.
The selectivity of cancer treatment can be improved by targeting molecules that are specific to cancer cells and not found on normal cells. These molecules can then be used as a target to antibody-based diagnostic or therapeutics or for drugs capable of altering their function.
The IkB proteins are a family of structurally related, intracellular proteins that bind to NF-kB and regulate its activity through a complex system of site-, and enzyme-specific phosphorylation, feedback gene regulation and translocation between the cytoplasmic and nuclear compartments. Several members of this family have been identified and include IkBα, β, ε, γ and BCL-3 (see Hayden & Ghosh, Genes Dev 18:2195-2224, 2004 for review of the NF-kB pathway). The sequence variability between the IkB proteins results in important differences in functionality.
IkBβ was initially identified as TRIP-9 and is also named NF-kappa-B inhibitor beta, NF-kappa-BIB, I-kappa-B-beta, IkappaBbeta, IkB-beta, IkB-B, thyroid receptor interacting protein 9 and TR-interacting protein 9. Two major splice variants, namely IkBβ1 and IkBβ2, have been reported. These isoforms differ in their C-terminal sequences with significant consequence on their degradation and effect on NF-kB. As a result, the β2 isoform appears more restricted to the cytoplasm and more slowly degraded than the β1 isoform (Hirano et al., Mol Cell Biol 18(5):2596-2607, 1998). The β2 isoforms was reported as the dominant form of IkBβ in cytoplasmic extract from the HT-29 human colon cancer cell line (km et al., Mol Carcinogenesis 29:25-36, 2000). Human IkBβ1 proteins have been proposed for the treatment of inflammatory and autoimmune diseases (U.S. Pat. No. 5,952,483) and a rabbit IkBβ protein analogous to human IkBβ1, for the treatment of disorders associated with NF-kB-induced gene activation (U.S. Pat. No. 5,597,898).
The NF-kB-IkBβ complex is generally retained in the cytoplasm of quiescent cells thus blocking the transcriptional activity of NF-kB. Upon site-specific and signal-induced phosphorylation of IkBβ, the complex dissociates and IkBβ is tagged for degradation through the ubiquitin-proteasome pathway. Free NF-kB translocate to the nucleus where it binds to DNA and regulate the expression of various genes (Malek et al., JBC 276(48):45225-235, 2001). The ability of IkBβ to retain NF-kB in the cytoplasm appears linked in part to the phosphorylation of the C-terminal PEST domain and the association of a further molecule with the complex (Chen, Wu and Ghosh, J B C 278(25):23101-106, 2003; Chu et al., Mol Cell Biol 16(11):5974-84, 1996) and thus varies between β1 and β2 isoforms. Newly synthesized or under-phosphorylated IkBβ may be able to translocate to the nucleus and retains the ability to bind NF-kB but without preventing or interrupting its gene transcriptional activity (Tran et al, Mol Cell Biol 17(9): 5386-99, 1998). In contrast, NF-kB is not strictly retained in the cytoplasm when bound to IkBα but moves back and forth between the nucleus and the cytoplasm through a complex import-export mechanism with an equilibrium that favors its localization in the cytoplasm of quiescent cells. The difference in the ability of IkBα and IkBβ to control the localization and activity of NF-kB is thought to be due in part to a protein insert found the IkBβ molecules but not the a form (Chen, Wu and Ghosh, J B C 278(25):23101-106, 2003). The role and specific regulation of the IkB proteins are far from being fully understood but it is generally accepted that these proteins are cytoplasmic, that some are able to translocate between the nucleus and the cytoplasm and that all are likely to have a significant role in the regulation of cell growth, apoptosis, response to inflammation and probably cancer.
Mutations of the IkBα gene in cancer cells have been reported (Cabannes et al., Oncogene 18: 3063-70, 1999) but to date, not for the IkBβ isoforms. Under- and over-expression of IkBα and IkBβ have also been implicated in disregulation of the NF-kB pathway in cancer cells (JBC 274(26):18827-835, 1999).